The proposed experiments investigate the development of the segmental pattern of cutaneous sensory innervation in skin of the hindlimb of the chick. Cutaneous axons from each dorsal root ganglion (DRG) innervate a characteristic region of skin, referred to as a dermatome. The objective of the proposed experiments is to determine whether DRG neurons are specified to innervate their dermatome prior to axonal outgrowth, and whether these neurons are directed to this skin regions by cues in the surrounding environment. As a first approach to this problem, the dermatome of each DRG supplying the hindlimb will be mapped at selected embryonic stages to determine the sequence of events during the normal development of dermatomes. The mapping will involve extracellular electrophysiological recording from each dorsal root, and labeling cutaneous sensory nerves with the marker horseradish peroxidase. In later investigations surgical manipulations involving skin transplantations, DRG deletions, and limb bud and spinal cord rotations will be made in embryos, and the resulting dermatomes mapped. The establishment of correct innervation by the DRGs in operated embryos will be taken as evidence that the cutaneous afferents are prespecified, and that the specification plays a role in the normal development of dermatomes. The failure of DRG neurons to innervate their correct skin region would suggest that during normal development the axons are directed to their characteristic region of skin by cues in the limb environment.